Multicarrier transmission schemes such as OFDM (Orthogonal Frequency Division Multiplexing), that is, schemes in which a multicarrier signal, including more than one sub-carrier to which more than one series of transmission data is allocated, respectively, is transmitted, may cause an effect of non-linear distortion in an amplifier (power amplifier) since the larger the number of sub-carriers, the larger becomes a Peak to Average Power Ratio (PAPR).
For example, according to the OFDM scheme, a plurality of phase coherent state of sub-carriers at a certain moment causes amplitude combination, thereby bringing about a power peak. At that time, if the peak exceeds a linear region of amplitude amplification in inputting to an amplifier, a non-linear effect is generated, so that out-band radiation is caused.
Hence, in previous techniques, when there is a possibility of generation of an amplitude value of a level at which a linear characteristic cannot be assured, processing called “clipping” which cuts out unallowable bits is used for guaranteeing the linear characteristic region of an amplifier.
For example, as schematically shown in (1) of FIG. 18, in a transmitter (base station or the like) employing the OFDM modulation scheme, the state in which data of multiple users are input from a core network (remote center) to a base station is expressed by P/S conversion (conversion into time series data) executed by the parallel/serial (S/P) converter 101. From there, data of a requesting user (selected by a non-illustrated scheduler) is read out, and is S/P converted into parallel time domain signals of the number corresponding to the number of sub-carriers by the serial/parallel (S/P) converter 102, and is then subjected to IFFT processing to convert the time domain signals into frequency domain signals for mapping the user data to each sub-carrier, which is then P/S converted (time-division multiplexed) by the P/S converter 104 before being output.
Here, when the signal after being subjected to the IFFT processing is restored into a time-domain signal by the P/S converter 104, a case can occur where an amplitude value of a level at which a linear characteristic cannot be assured is generated as shown in (2) of FIG. 18. Thus, as shown in (2) and (3) of FIG. 18, clipping processing which cuts out unallowable bits is performed to assure the linear characteristic region of the amplifier.
In this instance, as such a technique relating to peak suppression, other techniques are under study such as making contrivances in coding (coding is executed in such a manner that a PAPR is suppressed at signal points at which data is mapped to a sub-carrier, which technique is equivalent to performing mapping for suppressing the peak of a certain sub-carrier), PTS (Partial Transmit Sequence) transmission, and SLM (Selected Mapping) transmission.
The PTS transmission is a technique for suppressing the PAPR by means of appropriately performing phase rotation processing for each chunk of a sub-carrier block. Here, this technique requires a resource for notifying the rotated phase amount.
The SLM transmission is a technique in which multiple arithmetic operation circuits of the number corresponding to the number of sub-carriers are provided, and different random codes are multiplied on the time axis in different arithmetic operation circuits to select the most effective series for suppressing the PAPR. Here, this technique also requires a resource for notifying the selected series.
Further, as PAPR suppressing techniques, there also are techniques proposed in the following patent documents 1 through 3.
The technique described in the following patent document 1 has an object of suppressing peak power while suppressing deterioration of the transmission efficiency in multicarrier transmission. In order to accomplish the object, the peak power of a generated OFDM signal is detected, and upon detection of the peak power exceeding a threshold value, a signal for suppressing a peak power (for example, a signal of either one or both of whose amplitude and phase are limited therein) is inserted, in place of transmission data (information signal), into a specific carrier, out of multiple carriers, or an information signal of a specific carrier is punctured (removed).
The technique described in the following patent document 2 has objects of improving the transmission efficiency by means of providing different transmission quality for different items of transmission data in multicarrier transmission and further of suppressing peak power in multicarrier transmission. In order to accomplish the objects, there provided is an ability of recognizing code words formed by the n-number of signal points for 1-multicarrier symbol time, that is, it is possible to perform efficient transmission by means of performing coding different in the minimum code distance and sequentially changing the minimum code distance, thereby providing different transmission quality in one data frame. In addition, the above coding employs a peak power suppressing code as a code used in transmission, thereby making it possible to suppressing the peak power while providing different transmission quality.
The technique described in the following patent document 3 has an object of lowering a transmission peak power to average power ratio without deteriorating the transmission efficiency significantly. In order to accomplish the object, upon detection of a peak of an amplitude level not smaller than a threshold value, the peak cutting unit suppresses the detected peak down to the threshold value (that is, the already described clipping processing). After FFT processing to the signal whose peak suppressing has been performed, input data to be allocated to sub-carriers of fast transmission rates is substituted with a signal before being subjected to clipping processing for the purpose of protecting the input data from non-linear distortion due to clipping processing. As to this clipping, a case in which puncturing is performed is also considered.    Patent Document 1: Japanese Patent Application Publication No. 2001-339361    Patent Document 2: Japanese Patent Application Publication No. 2000-286818    Patent Document 3: Japanese Patent Application Publication No. 2005-101975